But the Orthodox church certainly teaches the Assumption. Denying something so clearly taught by the church is willfully wrong and surely jeopardizes salvation.
It is entertaining that you should say that, considering that in your very own Church, denying it was something which would not jeopardize one’s salvation before the dogmatizing of the Assumption in the 20th century (being a probable opinion beforehand). Regardless, within Orthodoxy, it is not a dogma like Homoousianism, Dyophysitism, Dyotheletism, etc. are.
The idea that certain wrong beliefs make salvation impossible, is interesting, but I think most churches, including the Orthodox churches, are more circumspect in discussing who is beyond salvation.
According to the all-holy gospel of Ecumenism, perhaps (and I notice that this false and heretical gospel has many followers these days). But the Fathers made it rather clear that heresy jeopardizes the salvation not only of those who preach it, but also those who accept it. This is why the Latin scholastics thought it better for the state to execute heretics who had fallen into heresy twice, rather than to allow them to propagate their heresies.
The idea that a belief makes salvation logically impossible in even more innovative.
Nonsense. That was the charge which all of the saints made against the heretics. St. Cyril, for example, made the charge that Nestorianism dissolved the entire economy of our salvation (see his 12 anathematisms). St. Gregory the Theologian objected to Apollinarianism for the same reason, causing him to write his famous line that, “what is not assumed is not healed.” St. Athanasius similarly objected to Arianism because an Arian Savior was incapable of truly saving man from His fallen state.
I think we would agree, however, that the Mystery of salvation cannot be constrained by the laws of logic.
But it can be expounded upon by the use of logic, and an incorrect exposition logically leads to incorrect conclusions about the economy of our salvation, which leads such incorrect expositions to be called heresies. The Fathers, in their wisdom recognized the need for formulae in order to rule out these incorrect expositions, and these formulae are what we call dogmas. The Dormition, in this sense is not a dogma, but rather a theological datum which we have passed down from the Tradition.